The present invention relates to a technology for controlling an automatic transmission including a fluid coupling such as a torque converter and, more particularly, to a technology for controlling a variable capacity type viscous coupling arranged in parallel relationship with the fluid coupling.
The characteristics of a vehicle provided with an automatic transmission such as the starting/running performances or the fuel economy are highly influenced by the characteristics not only of an engine but also of the automatic transmission, as is well known in the art.
For example, FIG. 26 is a diagram plotting the engine characteristics in terms of the relation between the rotational speed and the torque. A low speed type engine (E/G) has a higher torque at a lower speed side than 4,000 to 5,000 r.p.m., but a high speed type engine (E/G) has a higher torque at a higher speed.
On the other hand, FIGS. 27 to 30 are diagrams plotting the general characteristics of the torque converter. As could be seen especially from FIGS. 27, 28 and 29, the lower capacity type torque converter has the higher torque ratio at a lower speed ratio e at a start or acceleration so that it can produce the higher output torque with respect to an input torque.
If, therefore, a low capacity type torque converter is connected to a high speed type engine, the so-called "power run" can be achieved while the rotational speed of the engine is held at a reasonably high level by depressing the accelerator pedal. As could also be seen from FIG. 30, the low capacity type torque converter can utilize the maximum range of the engine torque so that starting acceleration is further improved.
In order to improve the mileage of the vehicle, on the other hand, it is preferable to use a highly efficient torque converter and accordingly a high capacity type torque converter. As could be seen from FIGS. 27, 28 or 29, the higher capacity type torque converter exhibits higher values in torque transmission efficiency when the speed ratio and the input rotational speed are at a somewhat high level. In an ordinary run with exception of a starting or extreme acceleration, the speed ratio is approximately at "1", and the engine rotational speed is about 2,000 to 4,000 r.p.m. so that the higher capacity type torque converter can create a more efficient run.
Here, what is required for the vehicle is preferably to satisfy neither the starting/accelerating performances nor the mileage in the aforementioned contrary relationship but to satisfy both of them simultaneously. Since, however, the characteristics of the torque converter are determined by capacity factors such as the external diameter of a pump impeller or turbine-runner, the structure of a blade or the structure of a stator, it is difficult for the existing torque converter to satisfy those contradictory characteristics.
In the prior art, therefore, a variable capacity type torque converter capable of varying the torque transmission capacity has been proposed in Japanese Patent Laid-Open No. 150066/1989.
This torque converter is enabled to change its performance curve by dividing a stator into a plurality of parts, by connecting the individual stator parts to a predetermined stationary portion by clutches, and by controlling the clutches. As a result, the torque transmission capacity can be stepwise varied in accordance with the degree of throttle opening.
In the torque converter disclosed in the aforementioned Laid-Open, however, the torque transmission capacity is varied by suitably releasing a clutch which holds any of the divided stators to eliminate a reaction of that stator. As a result, the torque transmission capacity to be set is restricted to the number of divided stators so that it cannot effectively function for all the various running modes. Moreover, it frequently occurs that the optimum point of the engine torque cannot be used.
If, on the other hand, the number of divisions of the stator is increased to eliminate such disadvantages, it is accordingly necessary to increase the number of one-way clutches for holding the stator parts directly and the number of clutches for controlling the engagement/release. Then, the structure is complicated to raise a problem in the difficulty of control.
In the prior art, on the other hand, the power loss to be caused by a slip in the torque converter is prevented to improve the mileage and the power performance by arranging a lock-up clutch in parallel relationship with the torque converter and by engaging it in a lock-up range having a vehicle speed at a constant or higher value.
With the lock-up clutch being engaged, the engine and the automatic transmission are in the so-called "mechanically direct lock-up" so that the vibrations due to the torque fluctuations of the engine are transmitted to the whole structure of the vehicle to deteriorate the riding comfort. Generally, therefore, the vibrations due to the torque fluctuations of the engine are absorbed by a slip of the torque converter by determining the lock-up range, by engaging the lock-up clutch when the vehicle running state comes into the lock-up range, and by releasing the lock-up clutch at a lower vehicle speed.
Generally speaking, the vibrations caused by the engine grow more serious with the higher throttle opening. If the throttle opening is higher to some extent, the lock-up clutch is released. With a relatively low throttle opening, however, the engine vibrations may increase, whereupon the lock-up clutch is subjected to a slip control.
One example of this control is disclosed in Japanese Patent Laid-Open No. 157859/1982. The system disclosed in this Laid-Open is a transmission system which is connected to a variable cylinder engine so that the slip percentage of the lock-up clutch is increased in a partial cylinder running mode having a reduced number of cylinders for combustions. In this partial cylinder running mode, more specifically, the torque fluctuations are increased to perform the torque transmission mainly through the torque converter by increasing the slip of the lock-up clutch so that they are absorbed or decreased to prevent the deterioration of the riding comfort.
Incidentally, the aforementioned variable cylinder engine is intended to improve the mileage under a light load. A similar engine is exemplified by an engine equipped with a lean combustion device. This device increase the air/fuel ratio under the light load and is equipped with a swirl control valve (as may be shortly referred to as the "SCV") in an intake passage for establishing intense swirls in the cylinders so as to stabilize the lean combustion.
In these engines, the output torque is highly varied in accordance with the change in the number of combustion cylinders and the open/closed of the SCV so that their characteristics are varied. As the engine having the variable output torque characteristics, there is known an engine which is equipped with a supercharge so that its output torque characteristics are discontinuously augmented unlike before if the supercharger is operated.
The vehicle in which the automatic transmission is connected to an engine having highly variable output characteristics is followed by various technical problems in accordance with the change in the output characteristics. In the invention as disclosed in the aforementioned Laid-Open, for example, the disadvantages caused by the fluctuations of the engine torque are eliminated by increasing the slip percentage of the lock-up clutch.
Unlike the aforementioned invention, however, the vehicle having its automatic transmission connected to the aforementioned engine is accompanied by serious problems when the output characteristics are changed to the higher torque. This point is not disclosed in the least in the aforementioned Laid-Open.
For example, here will be described the behaviors of the engine and the automatic transmission when the number of combustion cylinders is to be increased.
The engine capable of varying the number of combustion cylinders has a main object to improve the mileage in a light load state when the throttle opening is relatively small, as has been described hereinbefore. Thus, the relation between the output rotational speed in the partial cylinder running mode, i.e., the input rotational speed of the automatic transmission and the torque is plotted by dash lines in FIG. 31.
If, on the contrary, the full cylinder running mode is invited as the throttle opening increases, the relation between the input rotational speed and the torque is plotted by dash lines in FIG. 31.
If the engine has a small throttle opening and runs in a partial cylinder running mode and if the speed ration e of the torque converter at that time is at "0.5", this running mode is indicated by point A in FIG. 31.
If the accelerator pedal is depressed from this state to augment the throttle opening, the running mode shifts to point B of FIG. 31 while assuming that the speed ratio e is constant. If, in this case, the running mode is changed to the full cylinder running mode as the throttle opening increases, the output characteristics of the engine are discontinuously changed from the preceding lower torque ones to the higher torque ones so that the engine torque is discontinuously augmented. As a result, the rotational speed of the input member, i.e., the pump impeller of the torque converter is abruptly augmented to approximate the speed ratio e temporarily to "0", as indicated by point C in FIG. 31.
Since, in this state, the torque ratio of the torque converter rises, the turbine runner as an output member establishes a high torque to have an increased rotational speed so that the speed ratio e is returned to "0.5", as indicated by point D in FIG. 31.
If the engine torque is thus abruptly augmented, there occurs the so-called "temporary and abrupt rise of the engine rotational speed".
If this rise occurs, a driver generally releases the accelerator pedal to reduce the throttle opening so that the running mode is shifted to point E of FIG. 31. If, in this case, the running mode is shifted to the partial cylinder one as a result of the reduction in the throttle opening, the torque is further decreased, as indicated by point F in FIG. 31.
The operations thus far described will be described while stressing the actions of the driver. If the driver depresses the accelerator pedal to augment the torque, the engine torque is continuously augmented at first on the basis of the torque characteristics in the partial cylinder running mode. If, however, the throttle opening is large, the engine running mode is changed from the partial to full cylinder ones. As a result, the output characteristics are changed to the higher mode to augment the engine torque, and the torque ratio of the torque converter is temporarily augmented to increase the driving force abruptly.
This increase in the driving force exceeds the intention of the driver, and he instantly releases the accelerator pedal to reduce the throttle opening. The degree of release is sufficient to reduce the unintentional excessive driving force so that the throttle opening is considerably reduced. As a result, the running mode is changed from the full to partial cylinder ones. Since the torque reduction in this case is due to the discontinuous reduction of the engine torque, it may frequently exceeds the driver's intention so that the driver depresses again the accelerator pedal.
FIG. 32 plots the changes in the aforementioned throttle opening .theta., engine torque T.sub.E and vehicle driving force T.sub.A in time series. If the throttle opening is started at time t.sub.1 to increase to a predetermined value at time t.sub.2, the running mode is changed to the full cylinder one so that the engine torque T.sub.E and the vehicle driving force T.sub.A abruptly rise (at time t.sub.3). The throttle opening .theta. continuously rises to time t.sub.4, but the vehicle driving force T.sub.A is held substantially at a constant value as the torque ratio of the torque converter decreases.
Since the vehicle driving force T.sub.A thus abruptly rises, the throttle opening is decreased at time t.sub.4 so that the engine torque T.sub.E and the vehicle driving force T.sub.A are accordingly decreased (at time t.sub.5). Then, the running mode is changed to the partial cylinder one so that the engine torque T.sub.E and the vehicle driving force T.sub.A are further decreased within a short time period to time t.sub.6.
More specifically, if the number of combustion cylinders is increased with the increase in the throttle opening to change the output characteristics of the engine, the vehicle driving force T.sub.A extremely rises, as indicated by .DELTA. T.sub.A in FIG. 32, so that the drivers may feel physical disorder.
If the output torque characteristics changes, as described above, the unintended driving force is generated so that the driver has to frequently repeat the depressing and releasing actions of the accelerator pedal, thus causing a problem that the vehicle is difficult to drive.
Incidentally, the change in the output torque characteristics may occur not only in the aforementioned variable cylinder engine but also in the lean combustion engine or in the engine equipped with the supercharger. The above-specified problems may likewise occur in the vehicle in which the automatic transmission is connected to such engine.
In the torque converter having the lock-up clutch, on the other hand, the lock-up clutch is generally released, if the throttle opening is large, to prevent the vibrations due to the torque fluctuations of the engine. When, therefore, the engine torque is stepwise augmented with the increase in the throttle opening, the lock-up clutch is released. As a result, the torque transmission capacity of the entire transmission mechanism composed of the lock-up clutch and the torque converter is reduced to cause the abrupt rise of the engine rotational speed.